1. Field of the Invention
The present invention pertains to an improved method of measuring earth formation properties using a sensor disposed in a drill bit which detects electromagnetic radiation in the microwave frequency range so that the water content of the formation may be compared with the formation porosity to estimate the presence of hydrocarbon fluids in the formation.
2. Background
The continued development of so called wire line logging methods and the measurement of earth formation properties while drilling, sometimes referred to as measuring-while-drilling (MWD) and formation-evaluation-while-drilling (FEWD), has resulted in more useful data regarding earth formation properties than was previously available. The tools for performing these measurements are usually located in pipe sections or "subs" interposed in a drill string several feet above the drill bit. One of the advantages of making measurements with instruments which are disposed in the drill string near the bit is the decreased influence of the invasion of the formation by the drilling fluid or other fluids in the wellbore. In many formations, however, even the formation evaluation measurements taken only a few feet above the drill bit are subject to errors due to the effects of drilling fluid invasion into the formation material.
Another problem associated with present formation measurement-while-drilling technology is the inability of existing instruments to accurately measure formation properties of relatively thin layers or beds of earth material. Moreover, known types of instruments that do have relatively high vertical or wellbore longitudinal resolution also cannot investigate formation properties beyond relatively shallow depths in the formation material and hence these instruments see only the portion of the formation which has been invaded by wellbore fluids.
Accordingly, there has been a continuing and long felt need to develop a sensing device or system wherein relatively thin layers of earth formation may be measured with respect to certain properties and wherein the measurements taken are not subject to errors due to invasion of the formation material by wellbore fluids.